1. Field of the Invention
The present invention relates to an apparatus and method for compression and expansion of a wave signal on a time axis. More particularly, the present invention relates to an apparatus and method for compression and expansion of a wave signal on a time axis in which the compression and expansion of the waveform signal on a time axis can be performed smoothly.
2. Description of Related Art
In general, for example, the time axis compression and expansion technology (hereafter, referred to as xe2x80x9ctime stretch technologyxe2x80x9d as the circumstances warrant) has come to be utilized in the music production field as a technology for the reproduction of a waveform signal. In accordance with the time stretch technology, reproduction time of the waveform signal that has been recorded is compressed and expanded on the time axis.
For example, the rotation speed of the tape on the tape recorder at the time of recording on the tape may be made different from the rotation speed of the tape at the time of playing back the tape. This allows compression or expansion of the reproduction time of the audio signal that has been recorded on the tape on the time axis.
Let us assume that a waveform signal shown in FIG. 1 is recorded on the tape. When the rotation speed of the tape is reduced at the time of reproducing the waveform signal to a lower rotation speed than the rotation speed of the tape at the time of recording to thereby expand the reproduction time, the waveform signal shown in FIG. 1 is reproduced and changed to a waveform signal shown in FIG. 2, in which the waveform is merely proportionately expanded. As a result, the more the reproducing time is expanded, the more the frequency changes (the frequency lowers).
Therefore, in the conventional time stretch technology, waveform signals are successively, temporarily stored in a digital memory or the like, specified signal segments are culled or specified segments are repeated such that the reproduction time is compressed or expanded on the time axis.
Hereafter, the compression of the reproduction time on the time axis and the expansion of the reproduction time on the time axis will be simply referred to as xe2x80x9ccompressionxe2x80x9d and xe2x80x9cexpansionxe2x80x9d, respectively, as the circumstance warrants.
However, when a continuous waveform is subject to the culling or repetition, connection points between segment waveform signals at the time of culling and repetition become discontinuous. This results in another problem in that noises are generated.
In view of the above, a technique has been proposed to suppress the generation of noises and maintain the continuity of the plural waveforms by means of the cross-fading of the connection points of the waveform signals described above. The xe2x80x9ccross-fadingxe2x80x9d is a technique for continuously reproducing a plurality of waveforms, in which an end section of one of the waveforms (a xe2x80x9cfirst waveformxe2x80x9d) and a beginning section of another of the waveforms (a xe2x80x9csecond waveformxe2x80x9d) that immediately succeeds the first waveform are overlapped one on another. In addition, the loudness of the overlapping section of the first waveform is gradually decreased and, at the same time, the loudness of the overlapping section of the second waveform is gradually increased. However, this technique cannot completely prevent the fluctuation of the waveform signals or the generation of rippling, and therefore cannot provide a fundamental solution to the problems.
Furthermore, a waveform signal that is formed from a mixture of a plurality of different musical sounds has a relatively weak periodicity. Accordingly, the above-described technique suffers an additional difficulty in smoothly connecting the segment waveform signals at each connection section where the culling or the repetition of the waveform signal is performed.
Incidentally, it is known that a waveform signal having a mixture of plural musical sounds frequently has different waveform signal characteristics for different frequency bands.
In order to solve the problem of the discontinuity at each connection section of adjacent waveform signal segments when the culling or the repetition of the waveform signal is performed, one technique has been proposed. In this technique, a waveform signal that is subject to compression or expansion on the time axis is divided into main-frequency bands. The waveform signal of each of the divided frequency bands is independently compressed or expanded, such that the waveform signal is, as a whole, smoothly compressed or expanded on the time axis.
In this technique, a complex waveform signal that includes many overtones may be divided into as many frequency-bands as possible to provide smoother compression and expansion of the waveform signal on the time axis. However, when the number of bands (frequency bands) is increased, the amount of processing required for processing the waveform signal becomes substantial, and this makes it difficult to construct a low-cost system.
The present invention has been made in view of the problems of the conventional techniques described above. It is an object of the present invention to provide an apparatus and method for compression and expansion of a wave signal on a time axis that makes it possible to structure a low-cost system. This is achieved by dividing a waveform signal that is subject to compression and expansion on a time axis into main-frequency bands, and independently compressing and expanding the waveform signals of the divided frequency bands. As a result, the present invention suppresses an increase in the amount of signal processing of the waveform signals, that results from an increase in the number of bands (frequency bands), which may be required when a generally smoother compression and expansion process on the time axis is desired.
To achieve the object described above, an apparatus and method for compression and expansion of a wave signal on a time axis, in accordance with the present invention, uses a multi-rate sampling technique to thereby avoid an increase in the amount of processing required for processing waveform signals.
In view of the above, the present invention provides high-speed and high sound quality compression and expansion of a waveform signal on a time axis, using a multiple-rate signal processing technique that takes into account the auditory sense.
In accordance with one embodiment of the present invention, an apparatus for compression and expansion of a wave signal on a time axis comprises: a memory device that stores waveform data representative of a waveform for each sub-frequency band of each main-frequency band of a wave signal, in which the wave signal is divided into a plurality of the main-frequency bands, each of the main-frequency bands is divided into a plurality of the sub-frequency bands; a plurality of time axis compression and expansion devices, each provided for each of the sub-frequency bands for performing, based on the waveform data for each of the sub-frequency bands, time axis compression and expansion of the waveform that is represented by the waveform data; and a mixing device that mixes signals provided from the plurality of time axis compression and expansion devices. Each of the plurality of time axis compression and expansion devices performs compression and expansion in a process frequency that corresponds to one of the main-frequency bands to which the sub-frequency band subjected to the waveform time axis compression and expansion belongs, and wherein a process frequency of at least one of the main-frequency bands is different from a process frequency of at least one other of the main-frequency bands.
In accordance with one embodiment of the present invention, each of the main-frequency bands have the same number of sub-frequency bands.
In accordance with one embodiment of the present invention, the time axis compression and expansion devices for the waveform signal are implemented by software, and a program sub-routine for a time axis compression and expansion process in one main-frequency band is commonly used for a time axis compression and expansion process in another main-frequency band.
In accordance with one embodiment of the present invention, the time axis compression and expansion devices for the waveform signal are implemented by software, and the time axis compression and expansion devices for the sub-frequency bands that belong to the same main-frequency band use the same process program routine.
In accordance with one embodiment of the present invention, an apparatus for compression and expansion of a wave signal on a time axis comprises: a memory device that stores waveform data representative of a waveform for each sub-frequency band of each main-frequency band of a wave signal, in which the wave signal is divided into a plurality of the main-frequency bands, a waveform signal of each of the main-frequency bands is divided into a plurality of the sub-frequency bands; a plurality of time axis compression and expansion devices, each provided for each of the sub-frequency bands for performing, based on the waveform data for each of the sub-frequency bands, time axis compression and expansion of the waveform that is represented by the waveform data; and a mixing device that mixes signals provided from the plurality of time axis compression and expansion devices. The time axis compression and expansion devices for the sub-frequency bands that belong to the same main-frequency band perform compression and expansion in the same process frequency, and the time axis compression and expansion devices for the sub-frequency bands that belong to one of the main-frequency bands perform compression and expansion in a process frequency that is different from a process frequency for the sub-frequency bands that belong to another of the main-frequency bands.
The xe2x80x9cprocess frequencyxe2x80x9d of the present invention may be a sampling rate, for example.
These and other features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims.